get exception when consumer with 2.6.2 invoking provider with 2.7.x

[diecui1202]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: 2.6.2 as consumer and 2.7.x as provider
• Operating System version: mac os x
• Java version: 1.8

Step to reproduce this issue

1. start basic provider with 2.7.x
2. start basic consumer with 2.6.2

Pls. provide [GitHub address] to reproduce this issue.

Expected Result

The invocation get right result.

Actual Result

What is actually happen?

If there is an exception, please attach the exception trace:

java.io.IOException: Unknown result flag, expect '0' '1' '2', get 4
	at com.alibaba.dubbo.rpc.protocol.dubbo.DecodeableRpcResult.decode(DecodeableRpcResult.java:101)
	at com.alibaba.dubbo.rpc.protocol.dubbo.DecodeableRpcResult.decode(DecodeableRpcResult.java:113)
	at com.alibaba.dubbo.rpc.protocol.dubbo.DubboCodec.decodeBody(DubboCodec.java:89)
	at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.decode(ExchangeCodec.java:124)
	at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.decode(ExchangeCodec.java:84)
	at com.alibaba.dubbo.rpc.protocol.dubbo.DubboCountCodec.decode(DubboCountCodec.java:46)
	at com.alibaba.dubbo.remoting.transport.netty.NettyCodecAdapter$InternalDecoder.messageReceived(NettyCodecAdapter.java:133)
	at org.jboss.netty.channel.SimpleChannelUpstreamHandler.handleUpstream(SimpleChannelUpstreamHandler.java:80)
	at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:564)
	at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:559)

[diecui1202]

Since the corresponding PRs have been merged, so &READY-TO-CLOSE&